The invention comprises a method of inducting biaxial particulate alignment in a body of crystalline particles possessing anisotropic magnetic susceptibility, such as high temperature superconducting material.
Many High-Tc Superconducting Cuprates (HTSC) are known to have superconducting transition temperatures, Tc exceeding the temperature at which liquid nitrogen boil, 77K. As such they have a potentially large number of applications ranging from power generation, distribution, transformation and control, to high-field magnets, motors, body scanners, telecommunication and electronics. A high Tc value alone does not guarantee the utility of these HTSC at 77K or higher temperatures. Often these applications require large critical currents in the HTSC and this is not achieved unless the crystalline grains or particles of the HTSC are crystallographically aligned. This is commonly achieved in thin-films wherein the HTSC material is deposited on a substrate in such as way as to obtain crystallographic alignment of the material. However, thin film, while supporting very high critical current densities, Jc do not carry a very high absolute critical current, Ic because they are so thin.
Superconducting wires or other components which use bulk superconducting material can in principle support much higher Ic values provided they can be textured to achieve high Jc values. In processing HTSC to for such wires or similar, aligning the crystalline particles of the HTSC so that a major portion or ore of the particles have at least one similar axis parallel such as the c-axis, is commonly referred to as texturing the material. It has become apparent, at least for some and probably all HTSC, that crystallographic alignment along one common axis or monoaxial texture, may be insufficient to achieve high critical current density and the full biaxial texture in which two similar axes of the crystalline particles, such as the c- and b-axes, are aligned is preferable. An example of monoaxial texture of HTSC achieved by magnetic means is given by Tkaczyk and Lay (J. Mater. Res. 5 (1990) 1368) in which no significant improvement over unaligned material was seen.
It is known that such biaxial texturing or alignment can be achieved by linear melt processing in which the HTSC material is pulled slowly through a temperature gradient so that part of the material resides above the partial melting point and another part lies below, and the melt/solid interface is slowly displaced along the length of the material leaving behind dense, textured material in its path. This process, however, is difficult to control and extremely slow, producing biaxially textured material at a rate as low as 1 mm/hour. Linear melt processing is considered unsuitable for manufacturing long length wires exceeding 100 m or, worse still, 1 km.
Efforts have also been made to produce biaxial texture by a combination of monoaxial magnetic alignment and mechanical treatments such as pressing or rolling (Chen et al, Appl. Phys. Lett. 58 (1991) 531). However, the difficulty of achieving bulk alignment by mechanical means has prevented significant gains in critical current from being demonstrated.
The invention provides an improved or at least alternative method of inducing biaxial particulate alignment in a body of crystalline particles processing anisotropic magnetic susceptibility, such as high temperature superconducting material, which is useful as a step in forming bulk HTSC components such as wires, tapes, or other conducting elements, or other components.
In broad terms the invention comprises a method inducing biaxial particulate alignment in a body of crystalline particles having anisotropic magnetic susceptibility, so that at least a major portion of the crystalline particles have at least two crystalline axes generally parallel, comprising subjecting the particles to a magnetic field which varies cyclically relative to the body of crystalline particles with time and which has an average magnitude which is a maximum in a first direction, lower in a second generally orthogonal direction, and a minimum or zero in a third direction generally orthogonal to the first and second directions, to induce alignment of the axis of maximum magnetic susceptibility of the particles with the field direction of maximum average magnitude and the axis of minimum magnetic susceptibility with the field direction of minimum or zero average magnitude.
In one for of the invention a time varying magnetic field relative to the crystalline particles is produced by causing cycles of relative rotation between the body of crystalline particles and a magnetic field with the crystalline particles being subjected to the magnetic field in one orientation between the body of crystalline particles and the magnetic field for a time longer than the crystalline particles are subjected to the magnetic field in a second such orientation. The body of crystalline particles may be rotated relative to a magnetic field or the magnetic field may be rotated relative to the crystalline particles.
The relative may be cyclical between two positions with, in each cycle, the time at one position being less than the time at the other position. The angle of relative rotation may be between 0xc2x0 and 180xc2x0 and is preferably about 90xc2x0. Alternatively the relative rotation may be cyclical between three positions. The body of particles may be rotated within a magnetic field, between one position in which the body of particles spends most time so that the direction of the magnetic field relative to the particles in this position defines the direction of maximum average field magnitude relative to the particles, and two positions on either side of the maximum magnitude position, in which the particles spend less time. In these positions the angle of the field relative to the maximum average magnitude field direction may be between 0xc2x0 and 180xc2x0 and is preferably about 90xc2x0. For example biaxial alignment may be induced by cyclically rotating a body of particles about an axis first by 90xc2x0 with respect to an initial orientation in which the body of particles spends most time. The body of particles is then returned to the initial position and then rotated by 90xc2x0 in the opposite direction, and is then returned to the initial position and the cycle is repeated. The total time spend at the opposite positons (xc2x190xc2x0) is less than the time spent at the initial position so that the field direction when the body of particles is in the initial position defines the direction of the maximum average field magnitude relative to the particles. The average field magnitude along an axis parallel to the +90xc2x0 and xe2x88x9290xc2x0 positions is lower and the crystalline axis of intermediate magnetic susceptibility of the particles will tend to align along tis axis. The sign reversal of the magnetic field at opposite positions is unimportant for the aligning process as the potential energy of a body in a magnetic field of strength B varies as B2. Initial alignment may be accomplished by holding the body of crystalline particles in the maximum average magnitude position to induce an initial alignment of the crystals with their axis of maximum magnetic susceptibility along this field direction.
In another form of the invention the time varying magnetic field may be a net magnetic field which is the sum of a field in one direction and a field in a second direction, the strength of which field in the second direction varies with time. The direction of the first field relative to the crystalline particles (when the second field is switched off or at a minimum) will generally be the field direction of maximum average magnitude and the crystalline axis of maximum magnetic susceptibility will align in this direction. When the second field is switched on or at a maximum, it adds to the field in the first direction and the direction orthogonal to both the first and second fields is the direction of minimum average magnetic field magnitude, with which the axis of minimum magnetic susceptibility of the particles will align. Alternatively the field in one direction may be switched on while the field in another direction is switched off and vice versa. The field in one direction may be stronger than the field in the other direction or may be on a for a longer time in each cycle.
In another form of the invention a time varying magnetic field relative to the crystalline particles is produced by causing relative translational movement between the body of crystalline particles and a series of electromagnets or permanent magnets oriented alternately in one and the another direction. Either the crystalline particles may be moved relative to the series of magnets or a series of longitudinally arranged magnets may be moved relative to the crystalline particles. Either the body of particles may be moved more slowly past or stopped adjacent each alternate magnet, or alternate magnets may be loner in the direction of movement, or alternate magnets may be of higher strength. The net effect is that the particles are subjected to a magnetic field the direction of which varies with time relative to the particles and which has average magnitude which is a maximum in one direction, lower in a second direction, and zero or a minimum in a third direction, to achieve a common biaxial alignment of the crystalline particles. Instead of a series of magnets, a similar effect may be achieved by appropriate shaping of a magnetic core or by varying the density of windings of an electromagnet along the length of a core, to form a series of magnetic subfields.
The magnetic alignment may be carried out while the crystalline particles are settled through or suspended in an organic liquid, epoxy, polymer solution, molten wax, or other fluid, or a gaseous or liquid flow so that they are free to move under the influence of the applied magnetic fields. Where the crystalline particles are settled through a fluid they will form a sediment of aligned particles. Alternatively the suspension may have the property that, after alignment has been achieved, it can be arrested to preserve the texture, by solidification, curing, polymerisation, or chemical reaction, for example. Alternatively the fluid may be slowly drained off, or partially or completely volatilised or evaporated, or a suspending gaseous or liquid flow may be slowly ceased. Alternatively again the body of crystalline particles may be vibrated while subjected to the magnetic alignment, or the magnetic alignment may be carried out during electrophoretic deposition.
In a preferred form of the invention as applied to rare earth HTSC superconductor the particles used for alignment will be R2Ba4Cu7O15-xcex4 (where R is Y or a rare earth element) or RBa2Cu4O8 as these particles may be conveniently prepared as detwinned in contrast to RBa2Cu3O7-xcex4 which is typically twinned.
The magnetic alignment of the invention may be carried out using an electromagnet or electromagnets, a permanent magnet or magnets to generate the magnetic field, or using radio frequency or microwave means for example to generate electromagnetic fields.
The magnetic alignment may be carried out on crystalline material crystallising from a melt to induce biaxial texture in the resulting solid, by applying a time varying magnetic field to the crystallising material, and the term xe2x80x9cparticlesxe2x80x9d is to be understood accordingly, as including crystal nuclei for example.